This application is based on, and claims priority to, JP PA 2001-234320 filed Aug. 2, 2001, the contents of which are incorporated by reference. This application is also related to JP PA 2002-198186 filed Jul. 8, 2002, the contents of which are incorporated by reference.
1. Field of the Invention
The present invention relates to a semiconductor device such as a power semiconductor rectifier (power diode).
2. Description of the Related Art
Power diodes are used for various purposes. In recent years, power diodes have been used in inverter circuits, etc., that operate at relatively high frequencies of several kilohertz to tens of kilohertz. Power diodes used for such high-frequency operation are strongly required to have high switching speeds. Conventionally, pn diodes are mainly used as power diodes. Maintaining a necessary breakdown voltage using pn junctions, pn diodes have a smaller leakage current than Schottky diodes that maintain a necessary breakdown voltage using Schottky contacts. However, in pn diodes, too many minority carriers are accumulated in an n-type base layer during on-operation and, hence, it is necessary to sweep out accumulated carriers in a reverse recovery operation. Because the carrier sweeping-out takes time, the switching speed of pn diodes is low. To increase the switching speed (i.e., to increase the device operation speed), lifetime killers are introduced into the n-type base layer by diffusion of atoms of a heavy metal, such as gold or platinum, or by electron beam irradiation.
In recent years, power semiconductor rectifiers having an MPS (merged pin/Schottky diode) structure have been disclosed in which pn diodes and Schottky diodes are provided in parallel in a single chip. A planar-type MPS diode disclosed in Japanese Laid-Open Patent Application No. 31271/1985 has a large leakage current because the electric field strength cannot be made sufficiently low at the Schottky contacts. To solve this problem, structures in which trenches are formed have pn junctions formed at the bottoms of the trenches and, if necessary, at the side surfaces of the trenches. In these structures, Schottky contacts are formed on the surfaces of portions interposed between the trenches, as disclosed in Japanese Laid-Open Patent Application Numbers 63184/1993, 110062/1993, and 226638/1993. In a plan view, the trenches usually assume dot shapes (see FIG. 17(a)), and in some cases, the trenches assume striped shapes (see FIG. 17(b)).
Because there are no curved portions in the case of striped-shape trenches, the electric field strength in the p-type layers at the bottoms of the trenches in the active portion can be made lower than in the case of dot-shaped trenches if the ratio of Schottky contacts to pn junctions is small. On the other hand, if the Schottky ratio is increased, the depletion layer pinch-off effect between the trenches diminishes and, hence, the electric field strength increases in the p-type layers and in the Schottky contact portions, which results in a reduction in breakdown voltage and an increase in leakage current.
In the case of dot-shaped trenches, if the Schottky ratios are the same, the cell pitch (interval) can be made smaller and, hence, the leakage current can be made smaller than the leakage current in the case of striped trenches. However, if the semiconductor device is miniaturized by decreasing the cell pitch, the dot diameter decreases. As the dot diameter decreases, the curvature of p-type layers at the bottoms of the trenches increases, making maintenance of a high breakdown voltage more difficult.
As the openings of the dot-shaped trenches are made smaller, cavities become more prone to be formed when the trenches are filled with polysilicon. Therefore, with the dot-shaped trenches, it is difficult to increase the Schottky area ratio (about 90% or more) while keeping the leakage current small (i.e., while maintaining a high breakdown voltage).
An object of the present invention is to solve the above problems, thereby providing a high-speed, soft-recovery semiconductor device with an increased ratio of Schottky contacts to pn junctions and that does not have a reduction in breakdown voltage.
To attain the above object according to one aspect of the present invention, in a semiconductor device having an anode electrode that is formed on a first major surface of a semiconductor substrate having a first conductivity type, Schottky contacts are formed such that the anode electrode is selectively in Schottky contact with the semiconductor substrate, a cathode region is formed on a surface layer of a second major surface of the semiconductor substrate, and a cathode electrode is formed on the cathode region. A plurality of Schottky contacts are formed, and each of the Schottky contacts assumes, in the first major surface, a plan shape of a circle or a polygon whose apices are arranged on the circumference of a circle, with straight lines connecting the centers of the circles or the polygons adjacent to each other to form a triangular lattice unit.
Ring-shaped trenches having a prescribed width are formed in a surface layer of the first major surface of the semiconductor substrate. First Schottky contacts are formed such that the anode electrode is in Schottky contact with portions of the semiconductor substrate that are located inside the trenches, and second Schottky contacts are formed such that the anode electrode is in Schottky contact with portions of the semiconductor substrate that are located outside the trenches.
In another aspect of the present invention, the semiconductor device comprises insulating films that are formed on side walls of the trenches, first semiconductor regions having a second conductivity type that are formed to be in contact with the bottoms of the trenches, and conductive materials that fill the trenches and electrically connect the first semiconductor regions to the anode electrode.
In a further aspect of the present invention, the semiconductor device comprises insulating films that are formed on the side walls and bottoms of the trenches and conductive materials that fill the trenches and are electrically connected to the anode electrode.
In a further aspect of the present invention, in a surface layer of the semiconductor substrate, ring-shaped second semiconductor regions having a prescribed width and a second conductivity type are formed. First Schottky contacts are formed such that the anode electrode is in Schottky contact with portions of the semiconductor substrate that are located inside internal circles of the second semiconductor regions, and second Schottky contacts are formed such that the anode electrode is in Schottky contact with portions of the semiconductor substrate that are located outside outer circles of the second semiconductor regions. The first Schottky contacts have a plan shape of a circle or a polygon whose apices are arranged on the circumference of a circle.
In a further aspect of the present invention, third semiconductor regions having a second conductivity type are formed on side walls and bottoms of the trenches, and conductive materials fill the trenches and are electrically connected to the anode electrode.
In a further aspect of the present invention, a semiconductor device comprises an anode electrode formed on one surface of a semiconductor substrate. A plurality of ring-shaped trenches are formed in the one surface of the semiconductor substrate, wherein centers, on the one surface of the semiconductor substrate, of each of the trenches form vertices of a triangular lattice. A plurality of first Schottky contacts are formed between the anode electrode and portions of the semiconductor substrate located inside the trenches. A plurality of second Schottky contacts are formed between the anode electrode and portions of the semiconductor substrate located outside the trenches.
In a further aspect of the present invention, a method of forming a semiconductor device comprises forming an anode electrode on one surface of a semiconductor substrate and a plurality of ring-shaped trenches in the one surface of the semiconductor substrate. Centers, on the one surface of the semiconductor substrate, of each of the trenches form vertices of a triangular lattice. A plurality of first Schottky contacts are formed between the anode electrode and portions of the semiconductor substrate located inside the trenches, and a plurality of second Schottky contacts are formed between the anode electrode and portions of the semiconductor substrate located outside the trenches.
It is preferable that an internal diameter r1 of the trenches satisfy a relationship of r1xe2x89xa6about 10 xcexcm.
It is preferable that a length L1 of each side of a triangular unit of the triangular lattice satisfies a relationship of r1+Wtxe2x89xa6L1xe2x89xa6about 20 xcexcm, where Wt is a width of the trenches and r1 is an internal diameter of the trenches.
Preferably, the width Wt of the trenches satisfies a relationship of Wtxe2x89xa6about 2 xcexcm.
Preferably, a length variation of the three sides of the triangular lattice unit is within about 20% of the length L1 of each side.
An edge layer having the second conductivity type is preferably formed in the surface layer of the semiconductor substrate under an outermost circumference of the anode electrode.
A width Le of the edge layer preferably satisfies a relationship of Lexe2x89xa7r1+2 Wt.
Preferably, a diffusion depth Xje of the edge layer satisfies a relationship of Xjexe2x89xa7Xjt, where Xjt is a depth of anode layers at the bottoms of the trenches.
A shortest distance W1 between the edge layer and a closest trench preferably satisfies a relationship of W1xe2x89xa7L1.
The conductive materials are polysilicon materials and the top ends of the polysilicon materials are higher than the surface layer of the semiconductor substrate.
The semiconductor device preferably comprises insulating films and polysilicon portions on the top corners of each of the trenches.
These, together with other aspects and advantages that will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.